The present invention relates, in general, to method and apparatus for heating metal articles of magnetic material to an elevated processing or forging temperature.
In the metal forging art, it has been common practice for many years to heat billets or workpieces of magnetic material such as steel to their elevated forging temperature, e.g., around 2250.degree. F. in the case of steel workpieces for forging articles therefrom, by an induction heating process utilizing an induction heating coil energized by a high frequency electrical power supply. It is, of course, well known that ferromagnetic or so-called magnetic metals such as steels commonly employed for metal forgings undergo a transition, during the heating thereof to their elevated forging temperature of around 2250.degree. F. or so, from a magnetic state to a paramagnetic or substantially nonmagnetic state at the Curie point temperature of the metal which, in the case of such common forging steels, is generally around 1400.degree. F. or so. For this and other reasons, therefore, the induction heating processes heretofore employed for heating the steel billets or workpieces to forging temperature have generally involved the use of high frequency electrical power supplies of various frequencies for energizing the induction heating coil in order to thereby improve the overall efficiency of the induction heating process. Due to its lower cost per kilowatt, a high frequency power supply of a comparatively low frequency level was normally employed for preheating the magnetic metal billets or workpieces throughout and slightly beyond their magnetic state temperature range, where the depth of penetration of the workpieces by the heating flux generated by the energized induction heating coil did not affect the overall efficiency of the heating process. Such lower frequency inductive preheating of the magnetic metal workpieces to their nonmagnetic state was then combined with a higher frequency inductive post-heating of the preheated workpieces, within their nonmagnetic state temperature range, to their elevated forging temperature of around 2250.degree. F. to thereby improve the overall efficiency of the entire induction heating operation.
The normal overall efficiencies of induction heating processes such as heretofore employed to heat magnetic metal billets or workpieces to an elevated processing temperature such as their forging temperatures of around 2250.degree. F. vary from an approximate minimum efficiency of around three pounds per kilowatt hour to a maximum efficiency of around six pounds per kilowatt hour. While efficiencies of this level have been accepted within the industry for many years, an improvement in the overall efficiency of heating processes for heating magnetic metal workpieces to their elevated processing or forging temperature has been a much desired object.